warnings bookkeeping: count warning codes instead of keeping the warnings

[ThomasWaldmann]

What

add_warning() stored the message, its args and the formatting style of every warning emitted during a borg run in a global list, but all that is ever read back from it is the set of warning codes (get_ec() computes the final exit code from that).

For a BackupWarning, the args include the caught exception, and an exception references its traceback and thus the frames (with all their locals) of the code that failed. So every warning about a failed file kept e.g. the extract_item() frame - including the chunk data that was being written - alive for the rest of the run. On a full disk, every remaining file adds another chunk.

Measured with borg extract of 300 files of 2.5 MB whose writes all fail (ulimit -f, so each write gets EFBIG):

	max RSS
master	876 MiB (300 exception objects and 555 MiB of chunk bytes kept alive via their frames)
this PR	416 MiB (same as a normal extraction of that archive)

How

• keep only a Counter of warning codes; add_warning() takes just the code
• init_ec_warnings() drops its never-used warnings parameter
• get_ec() logs the per-code counts at debug level
• print_warning() passes only the code

Tests

• regression test at the print_warning_instance() level: the exception wrapped by the BackupWarning must be freed afterwards (checked via weakref). Fails on master, passes here.
• unit test for the get_ec() warnings logic (one kind of warning, mixed kinds, error precedence, reset)

Note: 1.4-maint has the same bookkeeping in helpers/__init__.py, so the same fix applies there.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.75%. Comparing base (a6c45bc) to head (ff8b5ba).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10331      +/-   ##
==========================================
+ Coverage   87.72%   87.75%   +0.02%     
==========================================
  Files         103      103              
  Lines       18816    18810       -6     
  Branches     2904     2905       +1     
==========================================
- Hits        16507    16506       -1     
+ Misses       1605     1601       -4     
+ Partials      704      703       -1     

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